The present invention generally relates to a cross coil type instrument, and particularly to a cross coil type instrument which can be applied to a revolution indicator indicating a revolution of an engine for an automobile, a speed meter, and so on.
A cross coil type instrument includes a movable permanent magnet around which a plurality of coils are cross-arranged. Currents based on an quantity to be measured pass through the coils, which generate a composite magnetic field. The movable permanent magnet having a needle is rotated due to the generated composite magnetic field.
A conventional cross coil type instrument is described with reference to FIG. 1. A movable permanent magnet 31 is fixed to a needle shaft 32. A needle 33 is fastened to an end of the needle shaft 32. Two coils 34 and 35 are cross-arranged at an angle of 90 degrees. A cosX drive circuit 36 supplies the coil 34 with a cosX drive signal I as shown in FIG. 2 where "X" is an electrical angle. A sinX drive circuit 37 supplies the coil 35 with a sinX derive signal II as shown in FIG. 2. The levels of the drive signals I and II change based on a quantity to be measured.
When power supply to the drive circuits 36 and 37 is turned OFF, the movable permanent magnet 31 rests so that the needle 33 is fixed to a position indicated at that time. For example, in the case where a cross coil type instrument is applied to an indicating instrument for indicating engine revolution, when an ignition switch is turned OFF, a needle of the indicating instrument is held at a position indicated just prior to turning the ignition switch OFF. For this reason, the cross coil type instrument requires a reset-to-zero mechanism which returns the needle (movable permanent magnet) to a zero position when power is turned OFF.
Conventionally, the reset-to-zero mechanism is formed by a permanent magnet (reset-to-zero permanent magnet), which is positioned in the vicinity of the movable permanent magnet. The reset-to-zero permanent magnet functions to absorb the movable permanent magnet. It is noted that force due to the reset-to-zero permanent magnet is always exerted on the movable permanent magnet 31 during normal operation. Thus, the rotation (or deflection) angle of the needle is not proportional to a change in quantity to be measured. In other words, the needle rotates non-linearly in response to a change in quantity to be measured. In addition, it is impossible to arrange indication scales at equal intervals.